It's a very hard choice. Overall I'd say Diagon Alley is the better pick as it is a little bigger and has more in the way of shops and experience, and Gringotts is awesome.
But Hogsmeade is great too and has Hogwarts which is the iconic part of Harry Potter. Also as Danielocha30 said some of the aspects of Diagon Alley, especially the Gringotts ride, focus on stuff from later in the series so might not appreciate it as well.
But you really can't go wrong with either. So might look at what else the parks offer outside of HP. IoA has the big roller coasters, water rides, and Spider-Man. While Universal has Springfield, Mummy, and Transformers. If all you really care about is the HP stuff though I'd really recommend a park hopper ticket to experience both sections and the Hogwarts Express (make sure to ride it both directions as the experience is different each way).